The NERPRC SPF program is productive and has remained pathogen-free for seven full years. The pregnancy rate in the recently completed 1996-1997 breeding season was high as many young adult females completed their second breeding season. All growth in the SPF colony since 1993 has been through internal recruitment. An indication of the success of the breeding program is the increasing proportion of the adult breeding colony composed of second generation SPF animals. In 1997 approximately 30% of the adult female breeders will have been born to SPF founders, a figure that we project will rise to 50% in 1998 with continued internal recruitment and retirement of senescent animals. Regular examination of all animals for antibody to the three target retroviruses and HBV is key to derivation and maintenance of SPF breeding groups. Established SPF groups are now screened semiannually unless more frequent testing is warranted by unexpected clinical illness in the group. The NERPRC SPF colony experiences high fertility with good maternal behavior and minimal fighting. Colony fertility averages 70%, with minimal losses from neonatal death. The NERPRC SPF colony exhibits excellent maternal behavior, with very small numbers of infants requiring hand-rearing because of rejection. As projected, the SPF colony is a vital resource for PHS-supported AIDS research at NERPRC. The main value of the colony is to provide animals for research. Large numbers of animals have been assigned to studies at NERPRC in 1996 and 1997, a figure that is projected to stabilize at 60 per year. In addition, healthy SPF adults in the colony are an important source for blood, bone marrow and other tissue specimens for approved in vitro studies. In 1997, the colony consisted of 388 macaques.